In a wireless communication system such as a mobile communication system, a plurality of base station devices each relaying a wireless link to a wired link is disposed. Wireless cells that are areas at which radio waves transmitted from those base station devices arrive are partially overlapped, thereby forming a wide range communication area. Communication services are provided to a mobile station present in the wide range area formed as stated above.
Many mobile communication systems includes a wireless cell switching function called “handover” so as not to break off a communication session even if a mobile station that is holding a communication moves among wireless cells. During the handover, the mobile station maintains the communication by repeatedly releasing a wireless link that is already established and establishing a new wireless link. As one type of the handover, there is known a technique of so-called soft handover used in a W-CDMA (Wideband Code-Division Multiple Access) mobile communication system. This technique is intended to prevent temporary blackout of a communication during handover by establishing the wireless link at the same time by a single wireless mobile station and a plurality of base station.
A base station control device that manages a plurality of base stations determines whether a new wireless link is established and whether an existing wireless link is released during the handover by using a measurement result of wireless qualities notified by the mobile station. A series of process accompanying this determination are roughly as follows.
First, the base station control device compiles a list of wireless cells qualities of which are to be measured by using information on a wireless cell to which the mobile station is connected. The wireless cells listed at this time include the wireless cell to which the mobile station is currently connected and wireless cells adjacent to the current wireless cell. The base station control device creates an adjacent cell list that is a list of listed wireless cells and notifies the mobile station of the adjacent cell list.
Furthermore, at the time of notification of the adjacent cell list, information about conditions for an event notification from the mobile station to the base station control station is also notified. The event notification is made by the mobile station so as to inquire on the base station control device about a final determination when the base station control device determines that it is necessary to release the existing link and to establish a new link as a result of measuring the qualities of the wireless cells. Therefore, notification conditions are prepared for the event notification according to situations such as the establishment or release of the wireless link, respectively. Moreover, the event notification includes a type of an event that occurs (establishment/release of the wireless link or the like) and quality information on surrounding wireless cells at that moment.
The base station control device transmits an instruction to secure or release network resources such as a wired band managed by base stations and to add or release a wireless link for the mobile station by using the event information notified by the mobile station. The mobile station establishes or releases the wireless link according to the instruction from the base station control device. A series of handover process is thereby completed.
A concept called “wireless cell adjacency relationship” plays an important role in the series of process. The wireless cell adjacency relationship means a logical relationship set among a plurality of wireless cells. The base station control device selects a wireless cell to which a new wireless link is to be established among the other wireless cells having the adjacency relationship with the wireless cell to which the wireless link is currently established. The mobile station cannot hand over the wireless link to a wireless cell that does not have the adjacency relationship with the wireless cell to which the link is established.
Generally, an administrator of a mobile communication system manually creates and manages a list relating to the wireless cell adjacency relationship as the adjacent cell list. If lack of registration occurs in the adjacent cell list, the mobile station that is holding a communication cannot execute handover, possibly resulting in break-off of a communication session or degradation of a communication quality. Such circumstances greatly disturb user-friendliness and possibly produce dissatisfaction of a user and discontinuation of a service agreement. It is, therefore, an important task for a common carrier to create and manage an accurate adjacent cell list.
Examples of a typical method of creating and managing the adjacent cell list include a method of measuring wireless qualities of wireless cells within a service area by a motor vehicle mounting a measuring tool and creating an adjacent cell list based on a wireless cell overlap situation. With this method, however, it is necessary to regularly drive the motor vehicle within the service area so as to follow up a change in the wireless cell overlap situation according to a change in a propagation environment or in a service utilization situation. This imposes heavier burden on the common carrier as the mobile communication system has become popular and the number of wireless base stations increases.
As a technique for dealing with the above-stated problems, there is known, for example, a technique described in Patent Literature 1 to be described later. The technique of the Patent Literature 1 is to measure time when a mobile station is connected to each base station, notifies a base station control device of base stations for which connection time is short and to skip handover to the base stations. By doing so, base stations that possibly cause frequent handover are deleted from an adjacent cell list.
{Citation List}
{Patent Literature}
    {PL 1} Japanese Patent Application Laid-Open No. 2003-289562 (FIG. 3).